The 'fortification clause,' Article XIX of the Washington Treaty, both complicated and benefited the USN in the Pacific. Senior naval officers in the 1920s were aware of the Russian experience of Port Arthur in 1904-05. A naval base too far from its sources of support and replenishment presented serious challenges going forward, but also, the lack of such bases presented its own challenges.There was a plan in Britain for a new base for a Far East Fleet to be built. Ceylon was too far back, Hong Kong too far forward. Australia and New Zealand lobbied for Sydney, but in the end, Singapore was chosen.
Note: there was an existing base at Singapore, but this was a brand new base to be built on the North Side of the Island on the Johore Strait.
This was in 1921.
When the Washington Treaty was agreed, there was a provision that no new naval bases be built East of the 110th Meridian, Singapore is at 103. Existing bases could be repaired to their existing standard, but could not be improved on.
So Britain could build a new naval base at Singapore, it was completed in 1938, but how did this affect the USN and the IJN??
The strategic concerns of the USN in Asia had been 1) the Open Door policy for trade, and 2) defense of the Philippines. The fortification clause affected both due to the distance of Asia from US naval bases on the West coast and in Hawaii. In the Philippines, Cavite near Manila was the only realistic base for the Asiatic Fleet (such as it was) west of 180 degrees longitude. Guam was too small; Subic Bay was too exposed, and by about 1925 it was increasingly the opinion on the General Board of the navy and at the Naval War College was that any of them was indefensible in the event of war with Japan.
The Washington Treaty prohibited improvements in the Philippines, including the interpretation that basing modern army or navy aircraft there was in contravention of the fortification clause. This was the genesis of the 'Mobile Base Project' (MBP). The MBP developed from the 1920s and up until the Japanese withdrawal from the treaty in 1936. By then it was a large part of War Plan Orange and, due to the indefensible situation in the Philippines, had replaced a fortified naval base in the western Pacific as the foundation of strategy for the USN.
The prohibition on basing aircraft west of 180 degrees (as was the interpretation of Article XIX) resulted in plans for larger aircraft carriers - though not enough of them - and of long range flying boats and tenders to get more aircraft into the operations of the fleet. Much of that had to wait until the demise of the Treaty, but plans were made that otherwise may not have been. The extensive fleet train and the huge modular floating dry docks of the MBP were unromantic, but they were essential to the success of War Plan Orange and its successors.
I have not come across comments by USN personnel concerning the fortification of Singapore, but when the RN asked for USN forces to be sent there, it was declined. It may not have been only the dispersal of force, but also the possible fate of the naval base far from support that went into that declination.
In some ways, the fortification clause benefited the US as the Treaty gave up fortification of places that could not be defended for a similar lack of extensive fortification in the Gilberts, the Marshals and the Mariana Islands. Japan did not begin much entrenchment in those islands until 1938-39, and important places such as Truk were not heavily fortified. Truk was mostly a large anchorage and a big supply dump than a fortified base. Not that the advance across the Pacific was without heavy loss and casualties, but Japan did not develop large air and submarine bases in these islands. The General Board worried about the fortification clause up until the Japanese withdrew from the Treaty, but I think the navy did have confidence that War Plan Orange was the correct strategy.